1. Field of the Invention
The present invention relates to a power conversion apparatus including a plurality of series connections each having a high-voltage side switching element and a low-voltage side switching element connected in series to each other, a common negative side bus to which one end of each of the low-voltage side switching elements is connected, the other ends of the low-voltage side switching elements being connected to the corresponding high-voltage side switching elements, drive circuits provided for the respective low-voltage side switching elements for driving the low-voltage side switching elements, and a common power supply to which the drive circuits are connected.
2. Description of Related Art
Japanese Patent Application Laid-open No. 2008-61290 describes, as such a power conversion apparatus, a three-phase inverter having a structure in which the grounds of the drive circuits corresponding to the low-voltage side switching elements (IGBTs) of U, V and W phases are used as the negative side bus. This three-phase inverter includes low pass filters (LC filters) interposed between each of the drive circuits and the power supply.
The LC filters are provided for suppressing variation of the ground voltage of each of the drive circuits due to voltage drop in the negative side bus caused by a current flowing through the negative side bus. Suppressing variation of the ground voltage makes it possible to suppress variation of the input voltage applied to the drive circuits from the power supply.
By the provision of such LC filters, variation of the ground voltage can be suppressed for each of drive circuits respectively corresponding to the three phases. However, when the three-phase inverter employs a structure in which, of three connection nodes of the negative side bus with the three low-voltage side switching elements, the nodes not in the middle in the direction along the negative side bus are connected with the negative terminal of the power supply, the following problem arises.
The voltage difference between the connection node connected with the negative terminal of the power supply and each of the other two connection nodes may increase with the increase of the inductance between the connection node connected with the negative terminal of the power supply and each of the other two connection nodes. Accordingly, when one of the three connection nodes which is not in the middle forms a reference point, variation of the input voltage of the drive circuit whose ground is at one of the connection nodes which is most distant from the reference point may be considerably large.
It would be possible to suppress the variation of the input voltage by increasing the filtering capacity of the LC filter interposed between the drive circuit and the power source, for example by lowering its resonance frequency. However, in this case, the size and cost of the LC filter are likely to be increased.